This invention relates generally to automatic control systems for a coal gasifier and more specifically to an automatic ash level control system for a fixed-bed coal gasifier.
In operation of a fixed-bed coal gasifier, it is necessary to maintain the ash level at the prescribed elevation. If the level drops too low, grate burnout is likely to occur. If the level rises above a nominal optimum height, then the devolatilization, gasification, and fire zones become shortened with subsequent loss of conversion efficiency and over temperature at the gasifier outlet. To control the ash level, the grate is rotated at a speed/duty rate such as to maintain the nominal ash level, but control is dependent upon the availability of a dependable and relatively accurate means of measuring the ash level.
It has been the practice to provide thermocouples in the arm of the bed stirrer to monitor the temperature profile of the gasifier bed. The motion of the stirrer is such that a helical scan of the bed temperature is produced. Since the stirrer both rotates and reciprocates, the signals from the thermocouples are transmitted by means of an EM/FM telemetry system. The bed temperature signal is used as a means for determining the ash level through the development of an ash level meter as described in U.S. patent application Ser. No. 434,021, filed Oct. 12, 1982, for an "Ash Level Meter For A Fixed-Bed Coal Gasifier" by George E. Fasching. It has been the practice to manually control the ash level through the operators constant monitoring of the ash level meter output. However, manual grate control, even though operated with considerable care and dedication, has been found to be unsatisfactory causing erratic gasifier performance and grate burnout.
Thus, there is a need for a system to automatically control the ash bed level in a coal gasifier.